The Migratory Ambit of the Hop Aphid and its Significance in Aphid Population Dynamics

Abstract

(1) Of about 320 species of aphids whose aerial distributions were examined, only sexual autumn migrants of the hop aphid (Phorodon humuli Schrank) originated from dense, isolated and persistent population 'patches'. (2) These source patches were hop gardens maintained in two small areas in southern England. (3) Patterns of aerial distribution were the same for both sources and for both males and gynoparous females. (4) Parthenogenetic spring alates migrating from native host plants were more evenly distributed and patches could not be isolated because, like all other aphid species, migratory ambits overlapped. (5) As expected, migration was not random, but directional orientation was negligible. (6) Evidence suggests that boundary layer migration, stratiform drift and cumuliform high level migration (L. R. Taylor 1965), were all used on different occasions. (7) The gamma regression models (R. A. J. Taylor 1979a) described the mean density x distance profile; the median distance travelled was 15-20 km and the 9500 limit was 100-150 km. The A-function probably has extreme parameter values. (8) The regression of log spatial variance on log mean density is identical for the autumn migration of males and gynoparous females; the summer migration of virginoparous females differs in intercept but not in slope.